This document discusses symmetric-key algorithms and provides details on stream ciphers and block ciphers. Symmetric-key algorithms use the same key for encryption and decryption. Stream ciphers encrypt bits or characters one at a time using a pseudorandom generator, while block ciphers encrypt fixed-length blocks of bits at once. The document also notes that symmetric-key encryption requires both parties to share a secret key, unlike public-key encryption.

1. Symmetric-Key Algorithm
Prof. Neerj Bhargava
Mrs. Shubha Chaturvedi
Department of Computer Science, School of
Engineering & System Sciences
MDS University Ajmer, Rajasthan

2. Symmetric-Key Algorithms
• Symmetric-key algorithms are algorithms for cryptography
that use the same cryptograph keys for both the encryption
of plaintext and the decryption of ciphertext.
• The keys may be identical, or there may be a simple
transformation to go between the two keys.
• The keys, in practice, represent a shared secret between
two or more parties that can be used to maintain a private
information link.
• The requirement that both parties have access to the
secret key is one of the main drawbacks of symmetric-key
encryption, in comparison to public-key encryption(also
known as asymmetric-key encryption).

3. Types of Symmetric-Key Encryption
Symmetric-key encryption can use either stream
ciphers or block ciphers.
Stream
Cipher
Block
Cipher

4. Stream ciphers
• Stream ciphers encrypt the digits (typically bytes),
or letters (in substitution ciphers) of a message
one at a time.
• In stream cipher, one byte is encrypted at a time
while in block cipher ~128 bits are encrypted at a
time.
• Initially, a key(k) will be supplied as input to
pseudorandom bit generator and then it
produces a random 8-bit output which is treated
as keystream.

5. Block ciphers
• In cryptography a block cipher is a deterministic
algorithm operating on fixed-length groups
of bits, called blocks. It uses an unvarying
transformation, that is, it uses a symmetric key.
• Block ciphers take a number of bits and encrypt
them as a single unit, padding the plaintext so
that it is a multiple of the block size.

6. ASSIGNMENT
Q. Differentiate between Stream and Block
Cipher?